Adeno-associated virus (AAV) vectors are ideal for the long-term treatment of metabolic defects. Unique features including non-pathogenicity, low-immunogenicity and stable expression have positively resulted in successful preclinical and clinical evaluation of this vector. One of the potential, yet, unexplored areas of gene therapy using rAAV is metabolic bone defects characterized by a reduction in the mass of bone per unit volume as in osteoporosis or insufficient mechanisms for anabolic bone remodeling as in spinal fusion and fracture. Although, currently available hormonal and drug therapies for osteopenia aim to prevent further bone destruction by osteoclasts, therapies directed towards increasing bone mass by increasing the event of osteogenesis will be greatly beneficial. Treatments to increase bone density by anabolic agents are limited due to ineffective delivery methods and a short half-life of the drugs and purified proteins. Thus, novel methods to induce sustained in vivo osteogenesis should improve the pathophysiology of the disease. Our earlier studies established long-term efficacy of rAAV-transduced mesenchymal stem cells (MSC) to selectively engraft to bone, repopulate and express a transgene in a mouse model. Preliminary studies pertaining to this application indicated that recombinant AAV-2 (rAAV) transduces human and murine MSC and osteoprogenitors in high-efficiency and that rAAV-mediated transfer of bone morphogenetic protein-2 (BMP-2) leads to their differentiation into osteoblast lineage. Thus, we hypothesize that autologous transplantation of culture-expanded MSC, transduced with rAAV encoding BMP-2 under the control of osteoprogenitor-specific promoters, will result in osteoblast enrichment and increased bone mass. In the current proposal, we will evaluate this hypothesis to: 1) Determine the engraftment of ex vivo cultured MSC and optimize enrichment of homing to bone by ectopic expression of a bone homing signal, 2) Determine bone-specific expression of BMP-2 and its osteogenic significance in vivo and 3) Determine the effects of AAV-mediated gene therapy in osteopenic mice models in vivo. A successful outcome of these studies may form the basis for future development of gene therapy approaches for osteoporosis and other osteopenic diseases in humans.